12 Tips For Getting a Better Green Screen Key

Since we're about a month away from the 2015 NAB Show, I thought it would be timely to look back at some of the useful information I came away with from last year's sessions.

In this post, I want to talk about green screen workflows. Those of you who have worked on green screen shoots before (and that's probably a majority of you) know how important it is to properly light your green screen and light and place your talent. Otherwise, the edges of your key will be blotchy and your talent will have a green glow.

I had the opportunity to attend Jeff Foster's class, in which he provided some extremely valuable information for setting up, lighting, and shooting green screens. By adhering to his tips, you can get a nice, clean key in post-production.

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Make Your Footage Look Better With One Simple Adjustment

In an earlier post I wrote about proper exposure and the recommended IRE levels for different cameras, including Arri, Canon C300, Canon 5D Mark III, and the Blackmagic Pocket Cinema Camera. In this post I want to write about my experiences while working with the Panasonic AF-100. 

Late last summer I took the AF-100 and the 5D Mark III into the studio to see if I could create a picture profile that most closely matched with the picture profile I was already using on the 5D. Rather than recap that test, let me just point you to this blog post I wrote after finishing the test.

Although initially pleased with the picture profile I created, the more I shot with it, the more I felt like something wasn't quite right. Whenever I would grade footage from the AF-100, I found that I couldn't hold the highlights as much as I wanted. It was very easy to overexpose a shot. I needed more gradual roll-off into the highlights.

In my earlier test, I started with a picture profile published by Abel Cine. So, I went back to the well once again to see what I could find. I dialed in a profile setting labeled RANGE. I also dialed in another profile setting I found in another forum, and labeled it FLAT. I took the camera outside and shot three quick tests, using the three different picture profile settings.

When reviewing the footage, I found that each profile gave me good dynamic range, but the RANGE profile in particular stood out to me because there seemed to be a more gradual roll-off into the highlights. But finding a picture profile I was happy with was only part of the equation.

Usually I keep my zebras at 75%, allowing only slight striping to occur on the cheekbones and nose of my subject. However, I've learned that 75 IRE is too high when shooting with the AF-100. Blooming can occur very quickly, which presents difficulty when trying to grade the footage in post. See the below frame grab as an example:

I shot this last November. It has not been color corrected or graded, so you're looking at the original flat image straight off the SD card. Notice the nose and cheekbones of the guy on the right. They are blooming and look a little too hot, meaning that these highlights will be tough to control in the grade. Compare him to the guy on the left, who looks more properly exposed. These are midtones and highlights that I can work with in the grade.

Now look at the waveform of the same shot:

The guy on the left is hitting right around 50 IRE, while the guy on the right is hitting around 75 IRE.

After looking at this particular shot, I decided to bring my zebra levels down to 60% so I can better monitor the exposure of my midtones.

If you have any tips or suggestions, please leave them in the Comments field. I'm also interested to know what picture profile you use when shooting with the AF-100. 

How to Expose Properly

Proper exposure is crucial if you want to retain as much information in your shot as possible. If you happen to overexpose the frame and clip the highlights, that information is lost. No amount of correction and grading will bring back lost highlights. Conversely, more can be done to salvage lost shadow detail, but the trade-off is more noise, grain, and artifacting (depending on the camera and how crushed the shot is).

This is why using a waveform monitor is so important while on set. It reads luminance values from black (value=0) to white (value=100). Proper exposure means that everything within your frame falls above 0 and below 100. A good rule of thumb is to keep your midtones around 75. This is why, when I use zebras on my EVF, I keep them set to 75. Then I open up the iris enough to see a small amount of zebras on the forehead and/or cheekbones of my subject. That tells me that the midtones are hitting right around 75 IRE.

Another great way, besides using the zebras, to ensure proper exposure, is to use a middle gray card. Where middle gray should hit on your waveform monitor depends on the camera. The following information comes from Jem Schofield of The C47:

  • Canon (for the C100, C300, C500) recommends that middle gray hit around 33 IRE.
  • Arri recommends 38 IRE, but 40 if you are shooting LOG.
  • Middle gray should fall around 50 IRE if you are using the Standard picture profile on the 5D Mk III; 40 if you are using the Neutral profile.
  • Middle gray should fall around 40 IRE if you are using the Blackmagic Pocket Cinema Camera in film mode. 

Be sure to leave your tips and suggestions in the Comments below.